1. Field of the Invention
The present invention relates generally to magnetoresistive devices, and more particularly to magnetoresistive sensors and heads that include a magnetoresistive element having an antiferromagnetic layer and two magnetic layers sandwiching a non-magnetic interlayer, as well as to a process for producing the magnetoresistive element.
2. Description of the Related Art
Recent magnetic disks capable of high-density recording require a highly sensitive read head. One example of such a highly sensitive read head in disclosed in Physical Review B, Vol. 43, pp. 1297-1300, xe2x80x9cGiant Magnetoresistance Effect in Soft Magnetic Multi-layered Filmxe2x80x9d, which shows a structure having two magnetic layers separated by a non-magnetic metal layer, and in which one of the magnetic layers receives an exchange bias magnetic field from an antiferromagnetic layer.
The above-mentioned thesis points out that resistance (R) in the multi-layered film has a component which changes in proportion to cos xcex8, where xcex8 represents the angle between the directions of magnetization of the two magnetic layers. This phenomenon is referred to as the giant magnetoresistance effect (GMR) or spin valve effect.
The giant magnetoresistive head works in different modes depending on whether a sense current flows along or across the layer plane (referred to as xe2x80x9ccurrent in the planexe2x80x9d or CIP mode and xe2x80x9ccurrent perpendicular to the planexe2x80x9d or CPP mode, respectively). The MR ratio in CPP mode is more than twice as high as that in CIP mode at room temperature.
Among highly sensitive read heads is a tunnel magnetoresistive (TMR) head, which has attracted attention in recent years. The TMR head utilizes a ferromagnetic tunnel junction due to a tunnel barrier layer held between two ferromagnetic layers. Its structure is disclosed in Japanese Laid-open Patent No. 103014/1992. The structure is quite similar to that of a giant magnetoresistive head in CPP mode, the only difference being the tunnel barrier layer acting as an insulating layer which replaces the non-magnetic metal layer in the giant magnetoresistive head.
The conventional giant magnetoresistive head in CIP mode has a magnetoresistive film (GMR film) 30 which is composed of a free layer 35, a non-magnetic metal layer (Cu layer) 40, a pinned layer 45, and an antiferromagnetic layer 50, which are formed one on top of another as shown in FIG. 16. The magnetization of the pinned layer 45 is pinned in the direction of element height by the exchange bias magnetic field from the antiferromagnetic layer 50. In general, the axis of easy magnetization of the free layer 35 is made parallel to the direction of the track width of the head.
The giant magnetoresistive head in CIP mode is produced by a process which consists of a first step of sequentially forming a magnetic shield layer 10, a magnetic gap layer 20, and a magnetoresistive (GMR) film 30 on a substrate 5, a second step of arranging permanent magnets 60 and electrode films 70 at both ends of the GMR film 30, and a third step of forming a magnetic gap layer 80, with a magnetic shield film 90 interposed thereunder.
The giant magnetoresistive head in CPP mode also has a magnetoresistive (GMR) film 30 which is composed of a free layer 35, a Cu layer 40, a pinned layer 45, and an antiferromagnetic layer 50, which are formed one on top of another as shown in FIG. 17. The magnetization of the pinned layer 45 is pinned in the direction of element height by the exchange bias magnetic field from the antiferromagnetic layer 50. The axis of easy magnetization of the free layer 35 is made parallel to the direction of the track width of the head.
The giant magnetoresistive head in CPP mode is produced by a process which consists of a first step of sequentially forming a magnetic shield layer 10 (which functions also as a lower electrode film) and a magnetoresistive (GMR) film 30 on a substrate 5, a second step of patterning the GMR film 30, a third step of arranging permanent magnets 60 at both ends of the GMR film 30, a fourth step of arranging insulating films 65 of Al2O3 such that they cover the permanent magnets 60, and a fifth step of forming an upper shield layer 90 which functions also as an upper electrode film. The insulating films prevent current from flowing into the permanent magnet.
The tunnel magnetoresistive head is quite similar in structure to the above-described giant magnetoresistive head in CPP mode. The only difference between them is replacement of the magnetoresistive film 30 by the tunnel magnetoresistive film 31 which is composed of a free layer 35, a tunnel barrier layer 41, a pinned layer 45, and an antiferromagnetic layer 50 as shown in FIG. 18.
Both the giant magnetoresistive film and the tunnel magnetoresistive film (collectively referred to as xe2x80x9cmagnetoresistive filmxe2x80x9d) have permanent magnet films arranged at both sides thereof. The permanent magnet film applies a longitudinal bias field in the direction of track width to the magnetoresistive film so as to reduce Barkhausen noise attributable to the magnetic domain structure in the free layer. The longitudinal bias field applied by the permanent magnet film is distributed as shown in FIG. 19, which shows weakness at the center of the magnetoresistive element (due to the effect of the shield film) and an increase far from the center through which the track width extends.
The magnetic domain structure, which causes noise in the free layer, tends to occur at both ends in the track width direction where there is a strong self-demagnetizing field. Therefore, it is desirable to apply a stronger longitudinal bias field to both ends of the free layer in the track width direction.
The ever-increasing recording density in magnetic recording requires the magnetic head to have a narrower track width and a higher reproducing sensitivity.
The result of reducing the track width of the magnetic head is a decrease in the distance between the permanent magnets arranged at both ends of the magnetoresistive element. Decreasing this distance means that the area receiving the strong longitudinal bias field in the free layer increases relative to the track width. In other words, the longitudinal bias field, which is applied to the center of the free layer through which the track width extends, increases with the decreasing track width of the magnetic head.
On the other hand, the increasing longitudinal bias field causes the rotation magnetization of the free layer to decrease due to the signal magnetic field at the center of the free layer through which the track width extends. This leads to a decrease in reproducing sensitivity. Therefore, it is desirable to apply as small a longitudinal bias field as possible to the center of the track of the free layer, so as to secure good sensitivity.
One possible way to improve the reproducing sensitivity is to reduce the thickness of the permanent magnet film, thereby decreasing the longitudinal bias field applied to the center of the free layer through which the track width extends. However, the reduction in thickness of the permanent magnet film results in insufficient longitudinal bias field at the ends of the free layer, which leads to the occurrence of magnetic domains, which in turn produces Barkhausen noise.
The foregoing presents difficulties in achieving both improvement in reproducing sensitivity and reduction in Barkhausen noise if the magnetic head has a narrow track.
The present invention was completed in view of the foregoing. Thus, one of the objects of the present invention is to provide a magnetoresistive element, a magnetoresistive head provided therewith, or a process for production thereof, with a high reproducing sensitivity of low Barkhausen noise even though the track width is very narrow. Furthermore, another one of the objects of the invention is to provide a magnetic recording apparatus equipped with the magnetoresistive head that may be combined with a magnetic recording medium for high-density recording.
One of the features of the present invention is in the structure of the magnetoresistive element utilized by the magnetoresistive head, which comprises an antiferromagnetic layer, a second magnetic layer, a non-magnetic interlayer, and a first magnetic layer which are stacked on one another, and a specific region with predetermined length provided at an end of the first magnetic layer in the track width direction. In the specific region, the thickness of the first magnetic layer is thinner than that of its central part.
At both ends of the first magnetic layer in the track width direction, there is a tendency for a specific magnetic domain structure that causes noise to be easily generated, due to the large self-demagnetizing field. The longitudinal bias field necessary to suppress magnetic domains in the first magnetic layer varies depending on the product of the saturation magnetization and the thickness of the first magnetic layer (xe2x80x9camount of magnetizationxe2x80x9d). In other words, the smaller this product becomes, the smaller the required longitudinal bias field becomes.
For this reason, the first magnetic layer is formed such that regions at both ends are thinner than at its central part, so that the effect of the longitudinal bias field can be increased at the end regions rather than at the center of the first magnetic layer. By this structure, the occurrence of magnetic domains at the end regions of the first magnetic layer can be efficiently controlled, without increasing the longitudinal bias field to be applied to the first magnetic layer.
Another way of decreasing the amount of magnetization at the end of the first magnetic layer is to reduce the saturation magnetization instead of the film thickness. Therefore, one aspect of a magnetic head to which the present invention is applied is in the structure of the magnetoresistive element that comprises an antiferromagnetic layer, a second magnetic layer, a non-magnetic interlayer, and a first magnetic layer which are stacked on one another, and with a specific region with a predetermined length provided at an end of the first magnetic layer in the track width direction. In each specific region, the saturation magnetization of the first magnetic layer is smaller than that at its central part.
Thus, the first magnetic layer is formed such that regions at both ends have smaller saturation magnetizations than its central part, to increase the effect of the longitudinal bias field at the end regions relative to the central part. By this structure, the occurrence of magnetic domains at the end regions of the first magnetic layer can be efficiently controlled without increasing the longitudinal bias field to be applied to the first magnetic layer.
These and other objects, features, and advantages of the invention will be set forth more fully in the following description.